Optical fibers, carrying modulated light signals, are one of the basic communication mediums. In order to allow flexibility in communications, an efficient switching method, which allows coupling different optical fibers to each other at different times, is required. Mechanical switching methods which physically move one of the fibers into alignment with one of a plurality of other fibers are relatively slow and expensive. Other switching methods include converting the light signal received from a first optical fiber into an electrical signal, performing electronic switching and then regenerating a light signal from the electrical signal into another optical fiber. This method is costly and is limited in the bandwidth it can carry and the switching speeds at which it can operate.
Another switching method uses the polarization of the light signal to selectively transfer an input signal to an output optical fiber.
In some implementations, an input light signal is decomposed into two orthogonally polarized beams, for example, by a birefringent element. The polarization of one of the beams is rotated, for example by a waveplate, and the two orthogonally polarized beams are directed in parallel through one or more controllable polarization beam splitters (PBS), which are controlled to direct the parallel beams to a desired output. The beams are then combined and forwarded to their destination. Switches of this type are described, for example, in U.S. Pat. No. 6,992,748 to Koh et al., titled “Scaleable and Mass Manufacturable OXC Using Liquid Crystal Cells”, U.S. Pat. No. 6,141,076 to Liu et al., titled “Spatial Light Modulators Constructed from Ferroelectric Liquid crystal Devices with Twisted Structure”, U.S. Pat. No. 6,519,022 to Xu et al., titled “Optical Routing Switch using Symmetric Liquid Crystal Cells”, U.S. Pat. No. 5,724,165 to Wu, titled “Fault Tolerant Optical Routing Switch”, U.S. Pat. No. 5,946,116 to Wu et al., titled “1×N Digitally Programmable Optical Routing Switch”, U.S. Pat. No. 7,224,860 to Zhao et al., titled “Multi-Port Optical Switches”, U.S. Pat. No. 6,175,432 to Wu et al., titled “Multi-Wavelength Cross-Connect Optical Network” and U.S. Pat. No. 6,275,312 to Derks et al., titled “Optical Routing Switch”, the disclosures of all of which are incorporated herein by reference in their entirety.
U.S. Pat. No. 6,452,702 to Wu et al., titled “N×M Digitally Programmable Optical Routing Switch” and U.S. Pat. No. 6,337,934 to Wu et al., titled “N×N Switch Array with Polarization Displacer”, the disclosures of both of which are incorporated herein by reference in their entirety, describe a switching array of N×N PBSs, which can be used to selectively direct the parallel beams of any one of N inputs to any one of N outputs.
U.S. Pat. No. 6,807,329 to Zalevsky et al., titled “Method and Device for Polarization-Based All-Optical Switching”, the disclosure of which is incorporated herein by reference, describes another switching approach using polarization control. The input beam is split into two orthogonally polarized beams, which are then directed perpendicular to each other to different controllable rotating elements (CPRs). The optical paths of the resulting beams are then combined. A cascading method is used to generate a 1×N or 2×N optical switch.
These devices suffer from losses and cross-talk and require improvement. In addition, in those embodiments which allow N×M switching, the size of the switch is relatively large degrading its performance or even making its use impracticable for some applications.